Sulfur removal fuels fuel cell future

The arrival of materials that prevent sulfur-poisoning of fuel cells working at high temperatures adds to hopes that fuel cells will make it into the mainstream as an alternative energy source.

Maria Flytzani-Stephanopoulos at Tufts University, Medford, US, has developed sorbent materials that reversibly adsorb hydrogen sulphide (H₂S) from the flow of gas that passes over a fuel cell’s anode. These cerium and lanthanum oxide sorbent surfaces can work at temperatures up to 800 ?C. 

Stephanopoulos has designed a small reactor where H₂S adsorption and sorbent regeneration take place, so that only cleaned fuel enters the fuel cell. The adsorption step happens very quickly, in less than one millisecond, Stephanopoulos told Chemistry World. ’Any regeneration gas mixture [unused gas from the fuel cell] can be used to sweep this adsorbed H₂S from the saturated sorbent surface; and it can do this also very fast.’

Removing sulfur from fuel cells is a problem, in particular in solid oxide fuel cells (SOFCs), said Stephanopoulos. ’With SOFCs that operate at temperatures in the range 650-900 ?C, the removal has to take place at similarly high temperatures to keep large heat exchangers out and not compromise the energy efficiency of any such system,’ she explained. ’Our technology frees the SOFC from having to work with natural gas or sulfur-free fuels,’ she said.

Frono Barbir of the United Nations Industrial Development Organization (Unido) said sulfur removal is important, but not the main problem in fuel cell development. He hopes that the dependency on fossil fuels, even in fuel cells, will eventually be eliminated. 

There are two major reasons for using fossil fuels in fuel cells, said Barbir, an  associate director at the Unido-International Centre for Hydrogen Energy Technologies, Istanbul, Turkey. ’Fuel cells may be a more efficient way to generate electricity from fossil fuels, and use of fossil fuels may bring fuel cells to the market sooner without waiting for cleaner fuels to become available,’ he said.

Barbir advocates using fuel cells ’wherever and whenever it makes sense, as soon as possible’.

Stephanopoulos said her materials are ready to be used widely. They are small and cheap enough to be incorporated into SOFC systems, and will solve ’the acute problem of sulfur poisoning,’ she said. Extensions to applications in proton exchange membrane fuel cells are obvious, she added, ’our sorbent would also work at low temperatures.’

Katharine Sanderson